As widely known, when compared with bias tires, radial tires have exceeding characteristics with respect to such as the driving stability and the abrasion resistance in high speed running. On the other hand, as automobiles have been increasingly improved with respect to their output capacity and their high-speed performance, it has been increasingly strongly demanded of radial tires that their high-speed durability should be further improved.
Damages or ruptures which radial tires may undergo when they are run at a high speed are caused mainly due to the phenomenon such that both lateral edge portions of a belt layer disposed in a tire tread region undergo a rising dislocation caused by the action of centrifugal force. To provide a radial tire structure which does not permit the above rising dislocation of belt layer edges to take place, conventionally it has been a practice to cover the outer periphery of the belt layer with a belt reinforcing layer of an organic fiber cord arranged in the circumferential direction of the tire. However, when a belt reinforcing layer comprising an organic fiber cord arranged in the tire circumferential direction is wound about the outer periphery of the belt layer as above, the two ends of the belt reinforcing layer are overlapped one on the other to produce a joint (a lap portion). This lap portion makes the tire uniformity and durability worse. This problem tends to be more conspicuous particularly when tires are mounted on a high speed car or a high power car. Then, it has been proposed not to produce a stepped lap portion by forming the above-mentioned belt reinforcing layer by spirally winding a belt-type bundle of a plurality of organic fiber cords: See for example Japanese patent application Kokai publication No. 61-60303, Japanese utility model application Kokai publication No. 61-15604, Japanese patent application Kokai publication No. 62-203803 and Japanese patent application Kokai publication No. 62-251203.
However, it has been ascertained that if a belt reinforcing layer of a spirally wound-up structure as above is provided in a radial tire having a tread pattern in which a main groove is rectilinear in the circumferential direction of the tire, then an original cord arrangement in the belt reinforcing layer in which adjacent cords are equidistantly arranged tends to become disturbed in a region beneath the main groove, whereby the high-speed durability of the tire tends to be lowered. This means that the merit of the spirally wound-up belt reinforcing layer that there is no stepped lap portion therein cannot be sufficiently demonstrated. The inventors of the present invention have conducted various investigations to determine the cause of the above problem, and as a result thereof, discovered that the above generation of disturbance of an original equidistant cord arrangement in the belt reinforcing layer of a spirally wound-up structure takes place on account of that when an unvulcanized tire is formed in a mold, a tread rubber is pressed by a main groove-forming rib of the mold to generate a movement or shift of the rubber, which causes the original equidistant cord arrangement to become disturbed or lost.
In view of the above, it is a primary object of the present invention to provide a pneumatic radial tire which maintains the merit of the belt reinforcing layer of a spirally wound-up structure and yet is free of a disturbance of the cord arrangement in the belt reinforcing layer.
It is also an object of the invention to provide a process for the production of pneumatic radial tires, which permits the merit of the belt reinforcing layer of a spirally wound-up structure to be fully exhibited and yet does not permit the cord arrangement in the belt reinforcing layer to become disturbed.